1. Field of the Invention
This invention relates to a method for forming or creating a solder ball on a substrate or circuit board (work), e.g., BGA (Ball Grid Array), etc.
2. Description of the related Art
FIG. 12 illustrates a perspective view of a BGA 51.
A plurality of solder balls 45 (bumps) is arranged on the BGA 51. The solder balls 45 (bumps) are arranged with an equal interval, i.e., a determined pitch P. As a method for forming the solder balls 45 (bumps), a known method of screen printing solder paste on an electrode of a work uses a printing mask and forms the solder balls (bumps) on the electrode by re-flow processing of the solder paste. Due to recent increases in the density of electronic parts and decreased pitch P of the solder balls (bumps), there are now more requests for reducing a size of an opening of the printing mask.
When the BGA 51 is attached to another substrate or circuit board by soldering, a height H of the solder balls (bumps) should be higher. If the height H of the solder balls (bumps) is higher, a difference in a height of the substrate or circuit board, to which the BGA 51 is attached by soldering, due to cambering and curbing of the substrate or circuit board can be better absorbed. A difference in a height on a surface of the substrate or circuit board due to cambering and curbing of the substrate or circuit board of the BGA 51 can also be better absorbed. Further, a difference in thermal expansion can be absorbed. Hence, all the solder balls 45 (bumps) can be electrically connected without failure. Therefore, an amount of the solder paste coated on the electrode of the BGA 51 must be increased as much as possible, and the height H of the solder balls (bumps) must be made higher.
FIG. 13 illustrates a procedure of forming the solder balls 45 (bumps) by screen printing according to the related art. An electrode 33 is provided on a work 31,and a resist 34 is provided around the electrode 33. An opening 23 of a printing mask 21 is provided to correspond to the electrode 33. When the printing mask 21 is placed on the work 31 and a squeegee 25 is moved in a direction of the arrow, solder paste 24 is printed on a surface of the work 31.
In FIG. 13, (a) shows a printing state, and (b) shows a state after printing when the printing mask 21is detached from the work 31. In FIG. 13, (c) shows a state in which the solder ball 45 (bump) is formed from the solder paste 24 by re-flow processing. For increasing a height H of the solder ball (bump) in (c), a thickness W of the printing mask in (a) must be increased. However, when the thickness W of the printing mask is increased, side walls of the printing mask and the solder paste 24 stick to each other more firmly. Therefore, when the printing mask 21 is detached from the work 31, the solder paste 24 is also detached from the work 31. As a result, the solder paste 24 does not remain on the work 31, and a printing condition becomes insufficient. When the thickness W of the printing mask is increased, it is possible to lower a viscosity of the solder paste 24 to solve this problem. However, when the viscosity of the solder paste 24 is lowered, the solder paste 24 spreads around the electrode 33 after screen printing and contacts with adjacent solder paste. This causes problems, e.g., a failure in an electric connection, generation of a solder bridge, etc.